What Do Clams Eat?

Clams - The All Purpose Bivalve

Clams are filter-feeders. Ocean clams feed at high tide, when the sand
in which they are buried is covered by water. At low tide, clams wait
for the water to return. Freshwater clams feed all the time.

Clams extend their siphons or “necks” to take in water for
oxygen and food. They exhale the water through the siphon after respiration
and removal of nutritious particulates (ingestion). “Food” to a clam is the
suspended particles present in the water of its habitat. Much of this mass
is made up of plankton, ubiquitous at the very top level of ocean water and
otherwise circulated through all but the deepest (intercontinental) sea
levels. For clams to eat, the water available to them must be in motion.

Clams respirate through the movement of water across their
gills, although some O2 intake takes place along the body mantle. Water is
propelled across the gills and the body proper by means of cilia, tiny
hairlike protrusions of protein. O2 intake is low, and estimated at under
ten percent of that present in the amount of total water processed.

What constitutes the biomass clams consume? Whatever is
floating nearby. Clams are indiscriminate feeders. Suspended particles may
include products of decomposition from dead sea animals, plant protozoans,
tiny pieces of seaweed and so on but it is microorganisms plants need to
build their bodies, which are almost 100% protein. Saltwater clams are not
fed when cultivated, and gather their nutrition from particles common to the
area in which they are berthed. Freshwater clams, as in a private tank, need
not be fed at all should other sea animals be present: they will take in
particles of fish food along with fish excrement and flaked-off skin cells.
Should the freshwater clams exist in isolation, cultivated “green water”
available at pet shops, along with dried seaweed containing spirulina is
good nutrition.

Not all substances taken in by clams are healthful to
humans. Common contaminants are the consequence of runoff from nearby
streets and homes into the clam beds. This runoff may contain petroleum
distillates, heavy metals, antifreeze, pesticides and other dangerous
chemicals. Heavy metals such as chromium will build up in a clam’s flesh
over time without killing it, leaving it dangerous for human consumption.
More often, seasonal microorganism blooms make clams unsafe to eat.
Dynoflagetllates, a type of single-celled photosynthetic animal are a
natural part of the plankton food chain. They are characterized by two
flagella, whiplike “tails” that permit limited locomotion. Dynoflagellates
are indigenous to the Atlantic and Pacific coasts of the United States. They
become hazardous only at times of “red tide,” coherent blooms which are
monitored each year by oceanographers. Should red tide infest clamming
grounds, the animals are no longer safe to harvest or eat. This is because
red tide dynoflagellates secrete a type of neurotoxin known as cegrotoxins.
These interfere with the movement of sodium ions (Na+) across human cell
membranes thus impairing nerve and muscle communication. In rare instances,
red tide infection results in paralytic shellfish poisoning (PSP). The
condition is curable with prompt treatment, but left untreated can result in
breathing problems and eventually death by asphyxiation.

Dynoflagellates are also known as the source of “cold
light,” bioluminescence sometimes observed by nighttime swimmers. This photo
effect results when the luciferin in dynoflagellates is broken down by
luciferase in the presence of oxygen and adeninotriphosphate (ATP), and the
dynoflagellate is disturbed by nearby motion. To the swimmer, faint
illumination, with a green, yellow or bluish cast follows arm and leg motion
through the water.

Non-fatal poisoning from contaminated clams is not unusual
in the US, particularly in summer months. Fortunately, most clam-borne
illness is caused by clams that have ingested septic runoff. Eating such a
clam is likely to produce a one to two-day bout with chills, violent nausea
and fever, but the condition is not life-threatening except to those with
impaired immune systems. Red tide poisoning can be fatal. If you have any
doubts about illness caused by eating bad clams, take the patient to the
hospital immediately. Clamming beds are tested each year by local
governments, generally at the town level per county rules. Red tide blooms
are closely tracked by organizations such as the Woods Hole (Massachusetts)
and Scrippts (California) Oceanographic Institutes. When a red tide bloom
approaches shore, warnings are posted by law. Some red tide blooms never
present a danger to humans, because they stay out at sea. In recent years,
red tide blooms beginning in the Gulf of Maine and spreading south and west
have become more of a concern. Whether this is due to an increase in
atmospheric CO2 over time (“global warming”) is not yet known.